1. Field of the Invention
The present invention relates to a liquid-droplet jetting apparatus constructed to jet (discharge) liquid-droplets of a liquid from a cavity unit by displacement of an active portion in a piezoelectric actuator.
2. Description of the Related Art
As a liquid-droplet jetting apparatus, there is an ink-jet head and the like. In Japanese Patent Application Laid-open No. 2004-291543 or the like, an embodiment of the ink-jet head is described which is constructed such that a jetting pressure is applied from a piezoelectric actuator to a cavity unit having nozzles so as to jet droplets of an ink (ink-droplets) from the nozzles. For example, in an embodiment disclosed in the Japanese Patent Application Laid-open No. 2004-291543, the cavity unit is formed in a substantially flat shape, and inside the cavity unit, ink supply channels, each of which is formed to range from one of pressure chambers, formed to open on one wide surface of the cavity unit, to reach one of nozzles formed to open on the other wide surface thereof, are provided for the nozzles respectively.
On the other hand, the piezoelectric actuator has a plurality of piezoelectric layers, individual electrodes provided for the pressure chambers respectively, and common electrodes each of which is arranged to cover the plurality of pressure chambers. In this piezoelectric actuator, areas of the piezoelectric layers, sandwiched between the individual electrodes and the common electrodes from thereabove and thereunder, are active portions which displace or deforms by a drive voltage applied between the individual electrodes and the common electrodes. Then, the piezoelectric actuator is stacked and fixed on the one wide surface of the cavity unit so that the active portions correspond to the pressure chambers respectively.
In the ink-jet head constructed in such a manner, displacement of an active portion changes the volume of a pressure chamber to thereby jet an ink filled in the pressure chamber from a nozzle. Therefore, to jet ink-droplets in a predetermined amount and at a predetermined speed, it is necessary to generate a predetermined amount of volumetric change in the pressure chamber.
With respect to the ink-jet head as an liquid-droplet jetting apparatus, there are tendencies to increase the degree of integration (densification) in a plane arrangement of nozzles and to decrease the plane area dimension of pressure chambers, so as to correspond to the miniaturization of the ink-jet head, the highly densified recording, and to the micronization of liquid-droplet in recent years. Accordingly, the reduction of the length of a channel (including a pressure chamber) needed for one nozzle not only makes it possible to realize the adaptation to the miniaturization of the ink-jet head and to the micronization of liquid-droplets, but also shortens an inherent cycle of a pressure fluctuation generated in the ink, thereby increasing a driving frequency of the jetting, which in turn is effective to realize the high-speed recording. However, this inevitably leads to the reduction in the plane area dimension of the active portions provided for the pressure chambers respectively, and thus it is necessary to increase the displacement amount of the active portions so that the volumetric change is applied, to the pressure chambers, in a predetermined amount by the active portions as a whole. Consequently, the drive voltage required for driving the active portions is needed to be set high. Further, the cavity unit is not a perfectly rigid body. Therefore, the displacement of active portion or portions is absorbed by the displacement of the cavity unit, causing a problem such that a predetermined jetting speed cannot be obtained without further setting the drive voltage higher.